マントル・ウエッジ部の誘起渦とマグマの発生

抄録

Through analyses of experimental simulations of the mantle vortex induced by downgoing slab, the following things are deduced. An effective vortex occurs in the low Q low V layer from 50 to 150m depth and a material at 130km level raises up to 70km level for 4 to 5 million years. In a turning zone where the flow of vortex turns to a direction of the downgoing slab, increment of shear strain per unit length of streamline is extremely large. When a material at lower level rises up, partial melt will increase. As the partial melt among crystal grains is connected in network within a certain extent, such network is called “magma network” in this paper. Until the material reaches the turning zone, the magma networks in it effectively collide each other to coalesce into larger ones. In the turning zone, each magma network is strongly sheared and its extent increases. Thus the turning zone itself may grow into a giant network of magma. Since flow quantity of the vortex across the turning zone is known by the above experiment, if the volume fraction of partial melt is 5%, a rate of the melt inflow in the turning zone is evaluated to be 5×10^-5 km³ /km/yr. Referring to that volcanoes in East Japan are arranged in a mean distance of 60 km along the volcanic front, the amount of 3 km³ melt should accumulate during 1000 years in the turning zone with the horizontal width 60 km. The melt squeezed out rises up easily through the turning zone to form a magma pool under the volcanic front.